Why do pools offer work for multiple header hashes?

Hi. I'm just starting to get into Ethereum mining and am trying to understand better how the pools work. I see concurrent jobs submitted to my miner with different header hashes and equal seed hashes. Are these from uncles or other chains? If I use the getWork call on my own geth or parity node, I only ever see one header hash at a time.

For example, within a few seconds of each other I get all these RPC calls: